The son of Libya's late leader Muammar Gaddafi has been kidnapped in Lebanon by militants demanding information about the fate of a Shiite cleric who went missing in Libya decades ago.

Hannibal Gaddafi appeared in a video aired on local Al-Jadeed TV saying anyone who has information about Imam Moussa al-Sadr should come forward.

Gaddafi appeared to have been beaten up and had black eyes but said in the video he is "in good health, happy and relaxed".

It was not clear when Gaddafi, who is married to a Lebanese woman, was kidnapped.

Al-Sadr, one of Lebanon's most prominent Shiite clerics in the 20th century, vanished along with two other people during a trip to Tripoli in 1978. Lebanon blamed the disappearance on Muammar Gaddafi.

Later, a senior security official said police collected Gaddafi from the north-eastern city of Baalbek where he was being held by the Shiite militants, whose affiliation was not immediately known.

The official said Gaddafi was being brought to Beirut.

Al-Sadr's 1978 disappearance has been a long-standing sore point in Lebanon. The imam's family believes he may still be alive in a Libyan prison, though most Lebanese presume al-Sadr is dead. Today he would be 87 years old.

Al-Sadr was the founder of a Shiite political and military group that took part in the long Lebanese civil war that began in 1975, largely pitting Muslims against Christians.

"I am with people who have a cause and they are loyal to their cause," Gaddafi, who is married to a Lebanese woman, said in the video. "We should respect their loyalty to their cause and at least give them the truth."

Al-Sadr was one of the pioneers of the Shiite movement that has become a force across the Middle East, spurred by the 1979 Islamic revolution in Shiite Iran.

Born in the Iranian holy city of Qom, al-Sadr came to Lebanon in 1959 to work for the rights of Shiites in the southern port of Tyre. In 1974, a year before Lebanon's 15-year civil war broke out, al-Sadr founded the Movement of the Deprived, attracting thousands of followers.

The following year, he established the military wing Amal - Arabic for "hope" and an acronym for the militia's Arabic name, the Lebanese Resistance Brigades - which later fought in Lebanon's civil war.

Since al-Sadr's disappearance, Libya has maintained that the cleric and his two travelling companions left Tripoli in 1978 on a flight to Rome and suggested he was a victim of a power struggle among Shiites.

Most of al-Sadr's followers are convinced Muammar Gaddafi ordered al-Sadr killed in a dispute over Libyan payments to Lebanese militias.

The Libyan leader was killed by opposition fighters in 2011, ending his four-decade rule of the North African country. Even after his death, al-Sadr's fate is still unknown.

Hannibal Gaddafi was arrested in 2008 for allegedly beating up two servants in a Geneva luxury hotel, sparking a diplomatic spat that dragged on for months. In 2005, a French court convicted him of striking a pregnant companion in a Paris hotel. He was given a four-month suspended prison sentence and a small fine.

He fled to Algeria after Tripoli fell, along with his mother and several other relatives.